Problem: Brandon is 4 times as old as Nadia and is also 21 years older than Nadia. How old is Brandon?
Explanation: We can use the given information to write down two equations that describe the ages of Brandon and Nadia. Let Brandon's current age be $b$ and Nadia's current age be $n$ $b = 4n$ $b = n + 21$ Now we have two independent equations, and we can solve for our two unknowns. One way to solve for $b$ is to solve the second equation for $n$ and substitute that value into the first equation. Solving our second equation for $n$ , we get: $n = b - 21$ . Substituting this into our first equation, we get the equation: $b = 4$ $(b - 21)$ which combines the information about $b$ from both of our original equations. Simplifying the right side of this equation, we get: $b = 4b - 84$ Solving for $b$ , we get: $3 b = 84$ $b = 28$.